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The occurrence and control of Phomopsis species causing Phomopsis symptoms and dieback in Western Cape Vineyards

by | Oct 25, 2020 | Viticulture

Project Number
P04000084

Project Title
The occurrence and control of Phomopsis species causing Phomopsis symptoms and dieback in Western Cape vineyards

Project Leader
Halleen, F

Institution
Stellenbosch University. Department of Conservation, Ecology and Entomology

Team Members
Halleen, F

Du Plessis, I

Moyo, P

Spies, C

Regnier, T

Vermeulen, C

Marais, J

Marais, D

Completion date

2019

EXECUTIVE SUMMARY

Objectives and Rationale
The aim of the project was to determine which Diaporthe (=Phomopsis) species are responsible for Phomopsis cane and leaf spot (“streepvlek”) and to determine if fungicides currently being used to control the disease are effective.
Objectives:
• To determine which species contribute to this disease by conducting detailed surveys of vineyards in the Western Cape Province with typical “streepvlek” symptoms, bleached canes, as well as dieback symptoms (Phomopsis dieback).
• To conduct surveys of other fruit and woody hosts surrounding vineyards to determine their status as alternative hosts.
• To confirm these findings by means of pathogenicity studies.
• To evaluate and implement effective control measures.

Methods
Vineyards were surveyed to determine the incidence of Diaporthe spp. associated with typical streepvlek symptoms, namely bleached canes, brown-black cracks which normally appear on the basal internodes of green shoots, as well as internal wood necrosis. Surveys of other fruit and woody hosts surrounding vineyards were conducted to determine their status as alternative hosts. Isolations were made from all these collected samples and the Diaporthe species identified by means of molecular analyses. Pathogenicity studies were conducted to determine the pathogenic status of these species. Based on the outcome of the surveys and pathogenicity trials,
representative isolates were subjective to fungicide screenings (in vitro) to determine their sensitivity towards fungicides currently being used. A field trial to test selected fungicides were also conducted in two vineyards.

Key Results
In total, 140 Diaporthe isolates were recovered from grapevine and 166 from 25 alternative hosts. A two gene (ITS and TUB) phylogenetic analysis revealed nine species from grapevine, namely D. ampelina, D. ambigua, D. cynaroidis, D. eres, D. foeniculina, D. novem, D. serafiniae, Phompsis sp. 5, as well as a new species described as D. nebulae. This is the first report of D. novem in South African grapevines and first report of D. serafiniae and D. cynaroidis on grapevines in the world. Thirteen Diaporthe species were identified from the alternative hosts, most of which have not been reported from their respective hosts in South Africa. D. ampelina did not occur on any of the alternative woody hosts sampled. However, several of the Diaporthe species occurring on grapevine were also found on other woody hosts. Several of the Diaporthe species were only found on other hosts and their status as possible grapevine pathogens is unknown. A survey of typical “streepvlek” symptoms (black-brown cracks on green shoots) conducted during the 2016/17 season in 36 vineyards in 18 regions representing 33 cultivars revealed that D. ampelina was associated with 100% of these symptoms and is undoubtedly the causal organism of Phomopsis cane and leaf spot. The bleached cane survey conducted during the 2017/18 and 2018/19 seasons in 108 vineyards in 15 areas and 18 cultivars showed that D. ampelina was associated with 100% of such bleached canes. Pathogenicity studies conducted as detached shoot assays showed that D. ampelina formed the largest lesions, but D. novem and D. nebulae also formed moderate to large lesions. In vitro fungicide screenings determined the EC50 values for 7 fungicides tested against D. ampelina. The two field trials showed that Folpan, Mancozeb and Maneb + zinc oxide applied to vineyards in a preventative strategy as prescribed provided excellent control against Phomopsis cane and leaf spot.

Key Conclusion of Discussion
Diaporthe ampelina is the causal organism of Phomopsis cane and leaf spot and bleached canes and only occurs on grapevines. Although several other Diaporthe species have been found form South African grapevines, they are associated with Phomopsis dieback which is another “form” of the disease which is managed in a different manner (i.e. these pathogens most likely infect pruning wounds where they cause internal necrosis and could be managed by applying pruning wound protectants). Fungicides currently registered and used in the South African grapevine industry are able to manage Phomopsis cane and leaf spot in vineyards if applied in a preventative spray program according to the manufacturers recommendations.

Recommendation to Industry / Key take-home message
Pruning debris, especially bleached canes which harbour Diaporthe fruiting bodies, must be removed from vineyards to lower inoculum levels (sanitation practices), or the debris must be chipped as fine as possible and completely worked into the soil. Sanitation practices should extend to other woody hosts surrounding vineyards to remove old wood and prevent other Diaporthe species from infecting grapevines and cause Phomopsis dieback. Fungicides registered against Phomopsis cane and leaf spot must be applied as recommended by the manufacturers, especially if it frequently rains during bud break and the early growth stages in spring. Phomopsis dieback must be treated as a grapevine trunk pathogen where pruning wound protection plays a cardinal role in an integrated strategy to combat the disease.

Final Report.pdf

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